In recent years, systems have been increasingly used that use biometric authentication, for example, in identity verification in withdrawing cash from an automatic teller machine (ATM), in individual identification at a security gate, and the like. The biometric authentication authenticates a person when input data, such as physical features including a fingerprint and a palm vein which are different from person to person, matches physical features registered in advance as data. Since the fingerprint, the palm vein pattern, and the like especially differ from person to person, a biometric authentication system in which advanced security is ensured through the biometric authentication is utilized by taking advantage of the fact that an appropriate use of a sensor and a computer enables automatic, highly precise, and valid identification.
In the authentication system described above, there are cases where registration data like ID as a collation target is not specified (or cannot be specified) because of convenience and the like of a user in collating each input data with a plurality of pieces of registration data in an authentication system and the like at a gate to be used by a number of registered users. In this case, collation between the input data and all pieces of registered data is needed and considerable time is required depending on the number of pieces of the registered data. However, it is only a small part of data that needs to be collated in detail and with precision in general, and majority of data can be eliminated in rough and high-speed collation processing. Based on this, multistage collation is generally performed in such a manner that the collation processing is divided into multiple stages, a rough and high-speed collating unit is used for refining data, and another collating unit which, although having precision, takes a long time for one collation is used for verifying the rest minority of data. An apparatus which performs the multistage collation is referred to as multistage collation apparatus.
In collation processing at each stage in the multistage collation, a precision and an amount (ratio) of data elimination at each stage are generally set and adjusted by parameters (a set of parameters) which are called as a threshold value with respect to a degree of similarity. When a threshold value at a stage is low, a possibility of mistakenly eliminating valid data is lowered in exchange for the fact that a refinement at the stage is lax, a ratio of registration data to proceed to a subsequent stage increases, and thereby a time for the processing in the subsequent stage increases. On the other hand, when the threshold value is set high, the possibility of mistakenly eliminating valid data increases in exchange for the fact that the refinement is performed severely, the ratio of registration data to proceed to the subsequent stage becomes small, and thereby the time for the processing in the subsequent stage becomes short. Thus, the threshold value needs to be adjusted precisely enough not to be too low and too high depending on a system to be used. Although the threshold value has generally been adjusted manually, since the adjustment operation requires time and cost including labor cost and the like, it is difficult to manually adjust the threshold value after the operation starts. Therefore, the parameters are generally adjusted before the operation and not changed during the operation.
However, since registration data is additionally registered momentarily, both the time and the precision of the collation momentarily change depending on a change in the number of pieces of registration data and a property of the registration data (how much amount of similar data is present and the like, for example) even during the operation of the multistage collation apparatus. Nevertheless, any breakdowns in the system including a failure in security maintenance, a prolonged collation, and the like are unacceptable.
Therefore, in the threshold adjustment, the threshold value is generally adjusted and fixed during the operation so that the collation processing is completed in a certain time period even in the worst case and the collation precision above a certain level can be obtained in accordance with assumed longest collation time and worst collation precision. On this occasion, the adjustment is performed with an assumption of a case where there are numerous pieces of similar data and a ratio of data to be collated at a subsequent stage at a lower speed and with a higher precision is heightened. Specifically, a method of: controlling an upper limit of the number of pieces of data to be registered so that the collation is completed in a certain time period even in the case where the collation takes time in proportion to the number of pieces of data (worst case); besides performing an adjustment in which a data elimination is not so actively performed at a previous stage for collation to be performed at a subsequent collation so that the precision is above a certain level (an error collation in which different pieces of data are regarded as being identical is below a certain level) when there are numerous pieces of similar data with the maximum registration data; and the like is adopted.
However, since the threshold value is fixed in accordance with the worst case in the threshold adjustment as described above, primary performance, which can be realized when an optimum threshold value is used, of the collation system cannot be obtained with respect to the number of pieces of registration data, registration data properties, and the like at each time point in the operation of the multistage collation apparatus, and thereby the system is operated with relatively low collation precision, at relatively low collation processing speed, and the like.
In view of this, as a unit for automatically adjusting a threshold value for collation at any given time, Japanese Laid-open Patent Publication No. 2006-59071 discloses a conventional technology in which, for example, a threshold value that draws a correspondence between a first value and a second value is calculated to perform biometric authentication based on this threshold value. The first value is obtained by dividing an absolute value of a difference between a threshold value associated with a living body and an average value of a plurality of first correlation values generated by detecting a correlation of a plurality of pieces of first characteristic data obtained in advance from the living body with data for collation, by a standard deviation of the plurality of first correlation values. The second value is obtained by dividing an absolute value of a difference between the threshold value and an average value of a plurality of second correlation values generated by detecting a correlation of a plurality of pieces of second characteristic data obtained in advance from another living body with the data for collation, by the plurality of second correlation values.
Japanese Laid-open Patent Publication No. 2007-213126 discloses another conventional technology related to individual collation. With the conventional technology, for example, first living body information is obtained from an individual as a target for individual collation. Then, second living body information and collation information indicating accumulated collation results in the past with respect to the second living body information are stored. A threshold value to be used for collation judgment is determined based on the collation information. A degree of similarity between the first living body information and the second living body information is compared with the determined threshold value for judgment.
Japanese Laid-open Patent Publication No. 2006-85268 has proposed still another conventional technology for effective multistage collation. With the conventional technology, for example, in a case with a degree of similarity apparently determined to be authentication failure among cases with various degrees determined to be authentication failure in the first collation, by controlling to perform only the second collation not to perform ineffectual collation processing or by updating registration data in the first collation with registration data in the second collation in a biometric authentication system of a multistage collation method.
Japanese Laid-open Patent Publication No. 2004-227589 has proposed still another conventional technology in which authentication is performed based on a combination of biometric authentication and a password. With the conventional technology, a threshold value to be used in biometric authentication is set to a first threshold value that lowers a false rejection rate (FRR), i.e., a probability of mistakenly rejecting a person as others, when an input password matches a registered password. Meanwhile, the threshold value to be used in the biometric authentication is set to a second threshold value that lowers a false acceptance rate (FAR), i.e., a probability of mistakenly accepting others as the person, when the input password does not match the registered password.
However, according to the conventional technologies described above, it is difficult to balance the collation precision and the collation processing speed in the collation processing between input living body information and registered living body information since an individual piece of living body information has a large amount of data and the number of registration is huge. It is also difficult to ensure the collation processing speed when priority is placed on the collation precision, and difficult to ensure security and the collation precision when priority is placed on the collation processing speed. Further, a user is required to input additional information including the password and the like, which reduces the convenience of the user.